I've been following this and the hundred other threads about this issue since I got my new receiver about 4 months ago and I have just today finally fixed my problem after trying nearly everything in this thread and others. I registered here to post what worked for me in case someone else runs into it.
This may not work for everyone but after speaking to Diamond, AMD and Onkyo and having each of them tell me it was the other persons problem I decided to try to flash my bios as one last ditch effort. Well i'll be damned, it worked.
I have a diamond HD7970 card which is a reference series. I flashed the AMD 7970ghz edition bios using the instructions at this page - http://icrontic.com/article/leaked-bios-easily-turns-radeon-hd-7970-into-ghz-edition
This also gave me the advantage of having the higher rated gpu and memory clocks as well as boost.
After it was all said and done I tested youtube using chrome which would constantly drop out after 5 seconds. No more dropping! Tested in a couple tv shows using MPC and had the same good result
I had tried everything before, even RMA'd both my onkyo ht460 and my 7970 only to have it continue to happen. If I tried it direct to my tv or to my denon receiver, hdcp was fine, but anytime it was plugged into the onkyo it would drop out.
I went ahead and updated to the 13.8 drivers just to check and it still works, I assumed it would as the HDMI audio driver is the same for 13.8 as it has been since the 12 series I believe. I had tried everything back to 11.9 since this started.
All of this being said, anytime you flash your BIOS will likely void your warranty and you do run a small risk of screwing things up if something goes wrong. I read up a lot about the bios flash before I did it and from everything that I've seen this will ONLY work on reference cards (ie. based on AMD's PCB) You should be able to tell if you have a reference by looking at the PCB itself, you will see a green serial number sticker and an AMD logo if its a reference card.
Hopefully this is of some help to some of you. I know I was really pissed off I was losing the ability to get high bitrate audio since I was having to use TOSlink instead of what I paid for.